1. Field of the Invention
The present invention generally relates to improvements in control of commodity distribution systems, e.g. an electric power distribution system, and more specifically to the use of intelligent autonomous nodes for isolating faulted sections of distribution lines, restoring service to end customers, improving circuit protection and allocation of system resources.
2. Description of Related Art
In general, a distribution system comprises one or more sources connected through a distribution network to one or more delivery points. As the commodity (material or energy) is transported through the network, abnormalities (e.g., faults) may develop that can lead to a disruption of the normal flow of the commodity or a loss of the commodity from the system. In order to help minimize the effects of these abnormalities, a distribution system will typically have nodes at various locations throughout the network which operate to monitor or control the flow of the commodity through the system. It is desirable to not only minimize the loss of the commodity when an abnormality occurs, but also to minimize the number of users who experience an interruption of the delivery of the commodity due to any abnormality. In order to reduce the loss of the commodity, the nodes in a system may have the capability to respond individually to system abnormalities without coordinating with other nodes. In such a system, nodes can prevent the commodity from flowing through the part of the distribution system where the abnormality exists. However, this system may interrupt service to more users than is absolutely necessary.
The power distribution systems for which this invention is most useful are generally of low to medium-voltage distribution feeders (ranging from approximately 4 KV to 69 KV) originating in power distribution substations and leading to the source of supply for end customers of an electrical supply utility or agency. Although the electrical principles governing operation of these feeders are identical to those governing the operation of the higher voltage generation and transmission systems, the methodologies for building, operating and maintaining the lower voltage systems are different. These methodologies are dictated by much larger quantities and geographical dispersion of distribution equipment, and by much lower quantities of electrical power supplied per mile of circuit. This creates requirements for lower cost, modular, standardized equipment, which can be installed, operated and maintained with minimal labor and human supervision.
Failures of the distribution feeder (faults) occur due to downed power lines, excavation of underground cable or other causes and are typically detectable by sensing excess (short circuit/overcurrent) current, and occasionally by detecting loss of voltage. In distribution systems, it is sometimes the case that a loss of voltage complaint by the customer is the means by which the utility senses the outage, responding by dispatching a crew to isolate the fault and reconfigure the distribution system. The typical devices for isolating these faults are circuit breakers located primarily in distribution substations and fuses located on tap lines or at customer transformers. The substation breakers are generally provided with reclosing relays that cause the breaker to close several times after the breaker has detected an overcurrent condition and tripped open. If during any of these “reclosures”, the fault becomes undetectable, service is restored and no extended outage occurs. Particularly on overhead distribution lines, temporary arcing due to wind, lightening, etc causes many faults. Thus, the majority of faults are cleared when the breaker opens and service is restored on the automatic reclose. Alternatively, after some number of reclosure attempts, if the overcurrent condition continues to be present, the recloser goes into a “lockout” state which prevents further attempts to clear the fault.
Other than manually operated switches, most distribution feeders have no other means to isolate a fault between the substation and the fuses, thus any failure of the feeder results in lengthy, costly, inconvenient and potentially dangerous outages. The primary exceptions to this involve the use of devices known as “line reclosers”, “interrupters” and “automatic line sectionalizing switches” or “sectionalizers”. These are automatically operated devices, well known to those skilled in the art, and are referred to categorically in this document as “fault isolating devices”. The term “sectionalizer” refers to a specific family of automatic, fault isolating devices described below, while the terms “sectionalizing” and sectionalize” are used to describe the process of isolating a faulted section of line, which can be performed by all of the classes of switches described above.
The “line recloser” is typically a pre-packaged, version of the substation breaker with reclosing relay. Line reclosers typically consist of a fault-break switching device with integrated current sensing, plus a control enclosure containing fault detection hardware, control logic, user interface module, and battery-backed power supply. When placed on the distribution line between the substation and customer loads, a line recloser is typically set up with fault detection settings coordinated to operate before the substation breaker trips and to correspondingly prevent the substation breaker from tripping. This has the effect of reducing the number of customers affected by an end of line fault. On very long feeders, the more sensitive settings can be used to protect the feeder from faults of a magnitude too low to be detected reliably by the substation circuit breaker. Multiple line reclosers can be placed on a distribution line in series, although it becomes increasingly difficult or impossible to coordinate their settings such that only the nearest recloser on the source side of the fault operates.
The “interrupter” is typically a pre-packaged breaker and fault relay without automatic reclosing capability. Interrupters are used primarily in underground power distribution systems.
The “automatic line sectionalizer” or “sectionalizer” is typically a prepackaged combination of a load-break switch used in conjunction with a device known as a “line sectionalizer control”. The sectionalizer senses current (and optionally voltage) such that the operation of the circuit and the source-side protective device can be monitored. The sectionalizer is configured to open its switch under certain circumstances when the circuit is de-energized after some number of pre-configured voltage losses have occurred within a brief time interval. The circumstances vary from product to product, but are always based upon sensing of conditions caused by faults followed shortly by voltage losses. Sectionalizers are designed to coordinate with the operation of the circuit's protective devices. Typical sectionalizers are devices such as the Cooper Power Systems Sectionalizer type GV or GW manufactured by Cooper Industries, Inc, or the EnergyLine Systems Model 2801-SC Switch Control manufactured by S&C Electric Company.
Various types of distribution automation systems have been developed to isolate faults and reconfigure the distribution system to provide service to the maximum number of end users. These types of systems include various combinations of centralized controls, distributed controls and intelligent autonomous controls. In such centrally controlled systems, each node may communicate with a central control location which gathers information from each node and coordinates a system-wide response. The central controller typically maintains a detailed map of the system topology, and this map must be updated whenever the system is reconfigured or new nodes are added. This can make such centrally controlled systems less reliable and more difficult and costly to implement and maintain. Additionally, for small systems with few nodes, the need to include a central controller can significantly add to the cost of the system. Furthermore, once an abnormality is rectified, the nodes typically must be transitioned to a normal state or to a specified state. Once the abnormality is corrected, it is generally desired to place the nodes in the original configuration or a specified configuration, at present this is typically done manually.
Intelligent, distributed control methodology is illustrated in U.S. Pat. Nos. 6,018,449, 6,111,735, 6,243,244 and 6,347,027. While these systems may be generally suitable to perform their intended functions, it is advantageous to determine how to optimally reconfigure a complex distribution circuit while preventing overloading of any portion of the circuit; i.e. allocation of system resources. This becomes particularly difficult in circumstances where the circuit branches out (bifurcates) such that multiple load-side switches could attempt to simultaneously pick up additional load and overload the circuit.